FIG. 1 illustrates the known principle of a flyback-type switched-mode power supply. The power supply comprises a primary side 101 and a secondary side 102, which are linked together through a transformer. An AC input to the primary side is rectified in a rectifier 103. A choke 104 and a capacitor 111 serve as a filter at the rectifier output. The primary current through a primary winding 105 of the transformer is regularly chopped with a switch 106 driven by an oscillator circuit 107. Intercepting the current in the primary winding 105 causes energy that was temporarily stored in the magnetic field of the transformer to discharge in the form of a current through the secondary winding 108 of the transformer. A diode 109 on the secondary side rectifies the secondary current and causes a secondary voltage to be maintained across a capacitor 110. The secondary voltage acts as the DC output voltage of the power supply.
The operation of a switched-mode power supply includes rapid changes in electric currents through inductive circuit elements, which in turn is a known source of electromagnetic interference (EMI). In order not to disturb the operation of other electric devices, and also in order to reduce the unnecessary loading of the components of the power supply itself, it would be advantageous to suppress the various sources of electromagnetic interference as much as possible. Two known and widely used approaches for EMI suppression involve the use of filter components in the power supply proper and its connections, and the use of conductive, shielding enclosures around power supplies and/or or their parts.